We are studying the hormonal basis for the initiation of maternal behavior in the rat using behavioral techniques for testing females under various experimental conditions and biochemical techniques for measuring the uptake and retention of tritiated estradiol and E2 nuclear receptor concentrations in selected brain regions, including the medial preoptic area, which has been implicated in the neural and hormonal regulation of maternal behavior, and the amygdala, which also plays a role. Non-pregnant, pregnanat and parturient females will be used and ovariectomy and hysterectomy performed to remove endogenous sources of hormones. Estradiol action will be correlated with E2 nuclear receptor counts and brain uptake of this hormone. Progesterone inhibition and facilitation of maternal behavior stimulated by estradiol will be investigated behaviorally and via e2 nuclear receptor studies in brain. The role of estradiol prepartum will be studied with the antiestrogen CI-628, also the possible role of oxytocin in estradiol-primed females. The long term aim of this study is to pursue in greater depth than we have thus far the hormonal mechanisms of maternal behavior in a species in which the hormonal pattern during pregnancy resembles closely that which occurs in women, most ungulates, many other small mammals andin most subhuman primates. Since we have shown that maternal behavior is organized into three phases, a hormonal phase, a nonhormonal paostpartum phase, and a transitional phase spanning parturition and immediately postpartum - this latter frequently called the "critical period" for maternal attachment to her newborn, our studies should increase our understanding of the hormonal aspect of this phase which is receiving so much attention in medicine, psychiatry, and psychological thinking in general.